It has been proposed heretofore to manufacture strips from extruded plastic which strips have had on them a series of transversely positioned upstanding ribs which form between them longitudinal channels, the strip being formed of indefinite length and having at one edge portion a first interengaging member and at another portion either at the opposite edge or inward from the opposite edge a second interengaging member, the arrangement being such that when such a strip is wound in a helical manner with the first enterengaging member locking into the second interengaging member a tubular object is formed which may be of any selected length and is of considerable strength and has many useful purposes such as the formation of pipes which are to be buried in the ground and which then are protected by the helical ribs around the periphery of the pipe.
This invention envisages certain improvements to helically wound structures of this type.
During research the applicants have found that the shape of the transversely positioned upstanding ribs is of importance for a number of reasons, one being to provide maximum protection of a tube so formed by means of the outwardly projecting ribs, another being to provide cushioning means by a selected shape of the ribs, another being to so shape the ribs that better interlocking between adjacent overlapping margins of the strip can be achieved particularly where a fluid seal is required, and these ribs can also be formed to provide added reinforcing by incorporating or being shaped to incorporate reinforcing members, or can be given advantageous shapes when used as sand screens or the like where particles must be prevented from blocking apertures through the wall of the pipe itself, and the present invention relates to various strip formations which achieve the required strengthening and improvement of the pipes formed from such strips.